Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2024879
Neutron-absorbinq materials
The invention relates to a neutron-absorbing material. It
is composed of ultrahigh molecular weight polyethylene in
which a boron compound, preferably boron carbide B4C, is
embedded.
In contrast to alpha- and beta-particles, neutrons do not
have a charge and therefore cannot lose energy by ener-
gization on passing through matter. Consequently, their
penetration power is extremely high. Neutrons are subject
exclusively to the action of the nuclear forces and are
scattered on atomic nuclei. According to the collision
laws, the energy releases to the body undergoing a
collision are the greater in such scattering processes,
the more similar the mass thereof is to the mass of the
colliding body. Therefore, a bundle of neutron beams,
which penetrates lead plates of several meters thickness
without significant attenuation, is very greatly attenu-
ated when passing through hydrogen-contAining substances
of a few cm thickness. On average, the energy is reduced
to l/e on collision with a proton, whereas the energy
release to atomic nuclei of higher mass is less, due to
inelastic collision. It is known from the literature that
on average 18 collisions are necessary in hydrogen and on
average 114 collisions are necessary in carbon in order
to brake a neutron down to thermal energy. ,hese thermal,
i.e. slow neutrons can then be completely absorbed by
elements of high cross-section, such as cadmium or boron.
In neutron absorption, binding energy is released in the
form of secondary gamma-radiation. It depends on the
absorber material and can be of considerable magnitude.
Thus, the gamma-radiation energy is 6 MeV in the absorp-
tion of neutrons by cadmium, 2.2 MeV in that by hydrogen
and only 0.5 MeV in that by boron.
- As the materials which protect against neutron radiation,
especially water and paraffins as well as plastics
cont~in;ng significant quantities of hydrogen, such as
2024879
polyethylene, polyesters and polyamides, are used.
Thus, according to the teaching of German Auslegeschrift
1,297,869, moldings of thermoplastic or thermosetting
plastics, in which the carbon/hydrogen ratio or the
residual atom/hydrogen ratio is in the range from 1 : 2.1
to 2 : 1 and the molecular weight of which is less than
200,000, are used for protection against gamma-radiation
and neutron radiation. Such plastics can be from the
classes of high- and low-pressure polyethylenes, poly-
propylenes, alkylene/propylene or alkylene/butylenecopolymers, polyamides and polyesters.
In German Auslegeschrift 1,162,694, a neutron-absorbing
material is described, in which granulated polyethylene
is embedded in a hydrogen-cont~ining liquid which remains
liquid or cures to give a plastic.
However, the known neutron-absorbing materials have
properties which restrict their applicability. Thus,
although plastics have a low density, their processibil-
ity frequently causes difficulties. Moreover, their
mechanical behavior does not always meet all requirements
and their heat resistances are frequently unsatisfactory.
The invention is based on the object of providing a
neutron-absorbing material which cannot only be processed
by conventional methods but is also mechanically strong
and resistant to thermal influences and has a low
density.
This object is achieved by a neutron-absorbing material,
in which boron is embedded in polyethylene. It is defined
by a mean molecular mass, measured by viscometry, of the
predominantly linear polyethylene of at least
2.5 x 106 g/mol.
ri ne~r polyethylenes having a mean molecular mass of at
least 2.5 x 106 g/mol and up to 1 x 107 g/mol are also
`~ ~ 3 ~ 202487 q
described as ultrahigh molecular-weight polyethylenes
(PE-UHMW). The molecular mass quantified above is under-
stood to mean the values determined by viscometry. A
method for measuring them is described, for example, in
CZ-Chemietechnik 4 (1974), 129 et seq.
The preparation of PE-UHMW is known. It can be carried
out by various processes. A proven process, which is
operated under low pressure with mixed catalysts of
titanium(III) halides and aluminum-organic compounds, is
described in German Auslegeschrift 2,361,508.
Ultrahigh molecular-weight polyethylene is distinguished
by a number of advantageous physical properties. Its high
wear resistance, its low coefficient of fraction against
other materials, its excellent toughness behavior and its
remarkable resistance to numerous chemicals should be
singled out.
PE-UHMW having molecular masses of between
2.5 x Io6 g/mol and 8 x 106 g/mol, especially
3 x 106 g/mol and 6 x 106 g/mol has proven particularly
suitable for the neutron-absorbing material according to
the invention.
In order to ensure that no long-lived radioactive iso-
topes are formed by the nuclear process taking place on
neutron capture, the polyethylene must be substantially
free of impurities. In particular, the compounds still
present from the preparation, which were used as cata-
lysts or constituents of catalysts, must not exceed a
content of 200 ppm by weight, preferably 150 ppm by
weight, relative to the polymer.
Furthermore, it is advisable to protect the PE-UHMW from
effects of heat, light and oxidation. Examples of com-
pounds, alone or in combination, which have proven
suitable as stabilizers are as follows: 4,4'-thiobis-(3-
methyl-6-tertiary-butyl-1-phenol), dilauryl
2024879
thiodipropionate, distearyl thiodipropionate, tetrakis-
[methylene-(3,5-ditertiary-butyl-4-hydroxy-hydro-
cinn~m~to)]-methane, n-octadecyl-~-(4~-hydroxy-3,5~-
ditertiary-butylphenyl)-propionate and glycol bis-[3~3-
bis-(4'-hydroxy-3'-tertiary-butylphenyl)-butanoate~. They
are in general added in quantities of from 0.1 to 0.2 %
by weight, relative to the total mixture. The addition of
antioxidants is important for the reason that polyethyl-
ene is oxidized in the presence of oxygen under the
action of gamma-radiation. It is then transformed into
low molecular-weight, waxy products, embrittles and loses
its extensibility.
As a further constituent, the novel material contains
boron in the form of boron compounds such as boric acid
(H3BO3). Boron carbide B4C has proven particularly suit-
able. Boron nitride is less suitable because of its
thermal properties. Mixtures of different boron compounds
can also be used, but a chemically homogeneous substance
is preferred. Boron carbide is used in the commercially
available purity. For use of the novel neutron-absorbing
material in practice, it is essential that it is homo-
geneous. It is therefore advisable to incorporate boron
carbide, which is as finely dispersed as possible, into
the polyethylene, i.e. boron carbide of a particle size
which corresponds to the size of the polyethylene
particles. It has proven advantageous to use boron
carbide of a particle size of from 10 to 200 ~m and
especially from 20 to 80 ~m. This has the result that no
segregation of the components occurs during the process-
ing of the material and no irregularities arise in itsstructure. Surprisingly, the outstanding mechanical
properties of PE-UHMW are hardly impaired by the addition
of boron carbide, and certain physical features, e.g. the
attrition behavior, are even improved.
The boron carbide content in the novel material depends
on the layer thickness in which it is used. It has been
found that, in the case of thin thicknesses of material,
Z024879
i.e. at layer thicknesses of up to 5 mm, the screening
properties are markedly improved with increasing B4C
content. At layer thicknesses above about 20 mm, an
increase in the B4C concentration in the material to more
than 1 %, relative to the material, hardly has any
further effect on the absorption behavior. At a given
degree of attenuation, the required layer thickness for
absorption of thermal neutrons can therefore be deter-
mined via the B4C content.
Allowing for the desired material properties, the prepar-
ation and the processibility of the novel material, it is
advisable to adjust the B4C concentration to values of
from 5 to 50 % by weight, preferably from 10 to 40 % by
weight and especially from 20 to 30 % by weight, each
relative to polyethylene cont~ining boron carbide.
The neutron-absorbing material of the invention is
prepared by homogeneously ~ixing the starting materials
PE-UHMW, boron compound and, if desired, additives in a
suitable mixer and subsequently sintering the mixture
under pressure at temperatures of from 180 to 250C,
especially from 200 to 230C. The sintering pressure is
from 5 to 10 MPa, especially from 8 to 10 MPa. Cooling is
also carried out under pressure, and 3 to 5 MPa, prefer-
ably 4 to 5 MPa, have proven suitable. The sintering and
cooling times depend on the thickness of the material and
on the filler content. Thus, the sintering time is, for
example, 5 hours for plates of 60 mm thickness, which are
composed of 70 % by weight of polyethylene and 30 % by
weight of B4C.
The novel material can be mechanically worked in a
conventional manner, for example drilled, milled and
sawn, and allowance must of course be made here for the
properties of the boron carbide, in particular its
hardness; it can be formed by pressing.
The invention is explained in more detail in the
- 6 - 202487~
following example.
ExamPle
For the irradiation tests, laboratory plates of PE-UHMW,
having a molecular mass of about 3 x 106 g/mol
~R~(Hostalen GUR 412) with 1, 5, 10, 20 and 30 % by weight
of boron carbide were prepared in different thicknesses
of 1,5,20 and 60 mm under standard conditions (pressure
on sintering 5 MPa, pressure on cooling 10 MPa, sintering
and/or cooling time depending on the thickness of mater-
ial and on the filler content). Unfilled PE-UHMW of the
same molecular mass and in the same dimensions was used
as a comparison.
-~3 The boron carbide used was the commercial product
TETRABOR F 280 from Elektroschmelzwerk Kempten GmbH,
~,
having a particle size of 22-59 ~m.
For preparing the laboratory plates, the particular
components were homogeneously mixed in a laboratory
mixer.
The samples were irradiated with thermal neutrons of an
energy less than 1 eV.
The neutron absorption coefficients were calculated by
the following equation:
~ tot X
o
where:
X thickness of the sample
~tot total absorption coefficient which contains all the
absorption components and scattering components in
PE and boron.
* ~e.--~A~
- 2024879
Io counting rate of the neutron beam, measured before
the use of every sample thickness, in order to
eliminate changes in the reactor power.
I attenuated counting rate at layer thickness X of the
sample.
The measured results show that, at low layer thicknesses,
the screening of thermal neutrons by the novel material
increases with increasing B4C content. At large layer
thicknesses, B4C concentrations above about 1 % by weight
(relative to the PE-UHMW filled with B4C) do not lead to
any improvement in the absorption behavior. Thus, the
attenuation is then independent of the boron carbide
concentration.
~ - 8 - 2024879
Material Zero Counting ~tot
thickness counting rate I (mm~l)
(mm) rate I~ with
S (particles/ sample
second) (parti-
cles/sec.)
Hostalen GUR 1.08 5270 4172 0.2164
5.25 5302 1881 0.1974
20.1 5269 264.3 0.1489
60.2 5243 11.303 0.1020
+ 1 % B4C 1.11 5263 4100 0.2250
5.24 5244 1490 0.2401
20.3 5309 73.15 0.2111
60.2 5249 1.155 0.1399
+ S % B4C 1.06 5252 3654 0.3424
4.76 5237 792.7 0.3967
19.8 5229 5.497 0.3464
58.8 5284 1.021 0.1454
+ 10 % B4C 1.16 5248 3029 0.4736
4.95 5276 357.1 0.5440
19.7 5236 1.573 0.4117
60.1 5298 0.919 0.1441
+ 20 % B4C 1.00 5270 2484 0.7522
5.02 5257 83.44 0.8253
21.2 5225 0.986 0.4045
60.3 5247 0.862 0.1445
+ 30 % B4C 1.03 5297 1904 0.9934
5.07 5202 28.87 1.0245
20.7 5231 0.966 0.4153
60.9 5298 0.856 0.1434
A comparison of the measured value shows that the absorp-
tion coefficients at small thicknesses are considerablyhigher than those at thicknesses of > 20 mm. This be-
havior can be explained by the fact that the thermal
neutrons are almost completely absorbed in thinn~r layers
and a small proportion of fast neutrons cont~ine~ in the
bundle of neutron beams is braked in thicker layers of
material to lower speeds and then absorbed. A substan-
tially small absorption coefficient must be expected for
these thermallized, originally fast neutrons.
- 2024879
In the table which follows, the particular material
thickness, at which 95 % of the thermal neutrons are
absorbed, is indicated in accordance with the equation
for calculating the absorption coefficients. This cal-
culation was based on the averages of the absorptioncoefficients for thin material thicknesse (s 5 mm) from
the absorption measurements for each B4C content.
Layer thickness~tot (mm~l)
(mm) at 95
absorption
PE-UHMW 13.6 0.22
(Molecular mass:
about 4 x 106 mol/g)
+ 1 % B4C 12.0 0.25
+ 5 % B4C 7-7 0 39
+ 10 % B4C 5.7 0.53
+ 20 % B4C 3.8 0.80
+ 30 % B4C 3.0 1.02
It will be clearly seen that the layer thickness is
considerably reduced with increasing B4C content.
